Non-aqueous electrolyte batteries, such as lithium ion batteries, have a high energy density and little self-discharge, and thereby they have been used more widely along with the demand for electronic apparatuses with higher performance and a smaller size. As an electrode for such a non-aqueous electrolyte battery, a spirally-wound body with a broad effective electrode area is used. The spirally-wound body is obtained by stacking a positive electrode, a negative electrode, and a separator, each having a strip-like shape, and winding the stack of them.
Basically, the separator serves to prevent the two electrodes from being short-circuited. Also, with a micro porous structure, the separator allows ions to permeate therethrough to cause a cell reaction. On the other hand, the separator having a so-called shutdown function (SD function) is used from the viewpoint of improving the safety. The SD function is a function to stop the cell reaction by deforming the resin thermally to close the micropores with an increased temperature inside the battery, when an abnormal current flow occurs due to an erroneous connection, etc. As the separator with the SD function, a polyolefin microporous film is known, for example.
However, conventional polyolefin microporous films have a safety problem because they have a low melting point and their mechanical strengths are lowered at a high temperature. In order to solve this problem, there has been proposed a method in which a polyolefin microporous film is stacked with a heat-resistant support body (JP 1(1989)-258358 A), and a method in which a resin component is crosslinked to increase the heat resistance (JP 63(1988)-205048 A), for example.
As the crosslinking system in the polyolefin microporous film, there have been known a method in which an active silane group is introduced to uncrosslinked polyolefin so that polyolefin is crosslinked by a crosslinking reaction between the active silane group and water (JP 9(1997)-216964 A), a method in which the polyolefin microporous film is heat-treated in the presence of oxygen (JP 2002-161165 A), a method in which the polyolefin microporous film is irradiated with an ionizing radiation, such as an electron beam (JP 10(1998)-7831 A), and a method in the polyolefin microporous film is irradiated with an ultraviolet ray (JP 2003-119313 A), for example. Particularly, the method of ultraviolet ray irradiation is used widely in the industries because it is fast, usable in a wide range of applications, and needs no large-scale facilities.
However, when polyolefin is highly crosslinked, the polyolefin loses its flowability at a temperature higher than the melting point of polyolefin, making the shutdown function insufficient. Thus, the degree of crosslinking must be limited to enhance safety, and there was a problem in using the highly-crosslinked polyolefin microporous film as the separator for a battery.
On the other hand, the separator for a battery also is required to have mechanical strength so as to prevent a through-crack from being formed therein by projections and depressions of the electrode and foreign matters. As the method for increasing the mechanical strength of the polyolefin microporous film, there have been proposed a method in which the polyolefin microporous film is stretched at a high stretching ratio (JP 2002-367590 A), and a method in which ultra high molecular weight polyethylene is mixed in the microporous film (JP 7(1995)-118430 A). However, in the microporous films obtained by these methods, polyethylene is stretched fully and comes to have a crystal structure. This causes a problem such that the melting point of the microporous film is raised and the shutdown temperature is raised accordingly, and the high melt viscosity of the microporous film hinders a prompt shutdown.
As described above, it was difficult to attain all of the shutdown function, the heat resistance, and the mechanical strength. In order to attain these properties sufficiently, there have been proposed methods in which a plurality of porous films with different characteristics are stacked (JP 6(1994)-98395 A and JP 11(1999)-329390 A). However, these methods have problems such that they require a complicated manufacturing process, and that they increase the thickness of the porous film, lowering the battery capacity per volume.